[Hectic]

Normally fields will give 5 - 10 fps variance max. That being said most 420 fields have that as a hard cap with 400 ish being the actual limit.
That being said, the easiest/cheapest way to reduce your fps would likely be to swap the inner barrel. Aside from that even just taking it apart (the hop up and barrel) and cleaning it and reassemble should help your consistency issues. After that actually using it for a few thousand rounds should settle the spring but maybe not by ~20fps that you need to be safe.
Edit: if you have the tools. Either a lathe or pipe cutting tools, you can just cut your own barrel. If you have "cnc" stuff you can port the barrel. Technically you can just cut it with a hack saw (gently and slowly) and then use a reamer on the end or even just some sand paper to finnish off the crown a bit in case of any edges.